Cement crack development due to thermal stresses because of poor dissipation of heat of hydration has long been recognized as a problem in the concrete industry, especially when massive structures, such as dams are being constructed. For example, European regulations in such cases require that the temperature increase should not be any higher than 36° F. (20° C.). Japan and Korea specify a temperature increase restriction for civil engineering projects by defining a Crack Index (ICr) in terms of ΔT (cross-section temperature difference in ° C.) as ICr=15/ΔT for internally restrained concretes. The greater ΔT, the smaller the ICr value, and the greater the probability of crack development. This is illustrated in FIG. 1.
During oil well cementing, the increase in slurry temperature due to heat of hydration can result in a temperature difference between the hydrating cement slurry and the wellbore fluids. This temperature difference between the hydrating cement slurry and the formation can easily exceed 15° C., resulting in ICr<1 and probability of crack occurrence>50%.
In the case of set cement, during the life of the well, the set cement is exposed to cooler temperatures on the wellbore side when cooler surface fluids are pumped into the wellbore, and hotter temperatures at the periphery due to high static formation temperatures. The temperature difference between the inner diameter (ID) and outer diameter (OD) of the cement sheath may reach on the order of several degrees. In the event that the cement is unable to respond to the temperature differences by expansion and contraction at an appropriate rate, or to quickly reach equilibrium temperatures by conducting heat due to its low thermal conductivity, cracks will develop in the body of the cement sheath. This also may result in casing contraction leading to debonding of cement from the casing and the formation of microannuli if the temperature is not equalized quickly.
Without wishing to be limited by theory, crack development during setting of cement due to heat of hydration may be explained as follows. When the temperature rise due to hydration is higher than the placement temperature, the cement paste will be in a compressed form due to volume expansion while being constrained by the wellbore wall and casing. During this phase, the hydrating cement paste has not gained sufficient strength to resist volume expansion and can relax by plastic deformation. When the exotherm of hydration is fully released, the temperature begins to decrease to the placement temperature. During this temperature drop, the built in stresses on the composition will transition from compressive stresses to tensile stresses. If the cement composition cannot accommodate the tensile stresses by optimal volume changes while being restrained, cracks will develop in the cement column. In another scenario, if the cement shrinks by debonding from the casing or formation to accommodate the tensile stresses, then a microannulus will form as a result. Consequently, there is a continuing need and interest to develop cement compositions with improved thermal conductivities